Photodetectors are devices used to receive a light signal and to generate an electrical signal indicative of the light signal intensity. Examples of photodetectors include a photodiode, a photomultiplier tube (“PMT”), an avalanche photodiode, and other light-detecting devices. Typically, the photodiode generates a current that corresponds to the intensity of light it receives. Photodiodes and other photodetectors typically use an integrating circuit to translate the current received into a voltage signal that is indicative of the intensity or power of the light over a period of time. Integration circuits are well known and operational amplifier (“op-amp”) circuits that integrate an input signal are well known. Integrator circuits have been employed to receive the photodiode current level and to generate a voltage value indicative of the integral of the current over a period of time.
Systems that employ photo detectors and integrator circuits are evolving into systems that are more general or open in implementation. That is, such measurement circuits are now required to process more general light inputs. Accordingly, the measurement circuits may need to provide signal-to-noise ratios (“SNR”) and dynamic ranges that are adequate for a variety of scenarios. In an absorption measurement system, a typical integrator circuit may not be sufficient to provide satisfactory SNR to exceed a 4OD (“optical density”) dynamic range over given wavelength range (200-1000 nm); particularly if weak flash lamps are used. The absorbance measurement systems may include optical components (light source, lenses, mirrors, gratings etc) in the light path for the light to be measured that have a varying optical behavior over the desired wavelength range. This variability may result in different SNR values depending on the selected wavelength.
Accordingly, the dynamic range of 4OD, which may often be the desired dynamic range, may only be reached using additional electronics such as, for example, programmable amplifiers and/or avalanche photodiode (APD) detectors. The use of additional electronics results in time consuming adjustment routines during manufacturing, which may also impact the flexibility of the system. In addition, APDs may be too expensive and overly sensitive to the ambient environment.
In view of the above, there is a need in the art for light measurement systems that permit variably fast adjustment of the light source output and of the dynamic range of the signal measurement.